Thickened nitromethane explosive containing encapsulated sensitizer

ABSTRACT

A liquid explosive primarily based on nitromethane contains small amounts of a sensitizer (an organic amine), a thickening agent, and up to 25 percent powdered aluminum. It may additionally incorporate ammonium nitrate, either per se or in an aqueous solution emulsified into the oily ingredients. Enough of the thickening agent is used to give the resultant liquid explosive low filtrate rate. Thus there is retarded tendency of the explosive to leak away into permeable earth formations. As a result it may be used in creating fractures in fluid-bearing formations and increasing drainage areas in wells. In fact, such a liquid may be used first without detonation to fracture a subsurface formation in which to place the major part of the explosive. It also has the very advantageous property of causing detonation to progress along the explosive even when it is in relatively thin cracks in the rock. The sensitizer is preferably encapsulated in quite small capsules of a solid slowly soluble in nitromethane and essentially insoluble in the sensitizer itself. The encapsulated sensitizer is added to the other components just before pumping into the well to minimize likelihood of premature explosion.

United States Patent 1 Fast 3,713,915 Jan. 30, 1973 THICKENEDNITROMETHANE [75] Inventor: Clarence R. Fast, Tulsa, Okla.

[73] Assignee: Amoco Production Company, Tulsa,

Okla.

{22] Filed: Nov. 23,1970

[21] Appl. No.: 95,005

Related US. Application Data [63] Continuation-impart of Ser. No. 3,511,Jan. 16,

1970, abandoned.

{52] US. Cl. ..l49/2, 149/91, 149/89, 149/38 [51] Int. Cl. ..C06b 19/00[58] Field of Search ..149/89, 91, 38, 92, 2

[56] References Cited UNITED STATES PATENTS 2.891,852 6/1959 Schaad..l49/89 3,309,251 3/1967 Audrieth et al. ..149/89 3,375,147 3/1968Sparks ct al. ..149/9OX 3,419,444 12/1968 Minnick ..l49/9O X PrimaryExaminer-Stephen J. Lechert, Jr. Alt0rneyPaul F. Hawley [57] ABSTRACT Aliquid explosive primarily based on nitromethane contains small amountsof a sensitizer (an organic amine), a thickening agent, and up to 25percent powdered aluminum. It may additionally incorporate ammoniumnitrate, either per se or in an aqueous solution emulsified into theoily ingredients. Enough of the thickening agent is used to give theresultant liquid explosive low filtrate rate. Thus there is retardedtendency of the explosive to leak away into permeable earth formations.As a result it may be used in creating fractures in fluid-bearingformations and increasing drainage areas in wells. In fact, such aliquid may be used first without detonation to fracture a subsurfaceformation in which to place the major part of the explosive. It also hasthe very advantageous property of causing detonation to progress alongthe explosive even when it is in relatively thin cracks in the rock. Thesensitizer is preferably encapsulated in quite small capsules of a solidslowly soluble in nitromethane and essentially insoluble in thesensitizer itself. The encapsulated sensitizer is added to the othercomponents just before pumping into the well to minimize likelihood ofpremature explosion.

2- Claims, N0 Drawings THICKENED NITROMETIIANE EXPLOSIVE CONTAININGENCAPSULATED SENSITIZER CROSS-REFERENCES TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of my application Ser. No. 3,51l,filed Jan. l6, 1970, now abandoned.

BACKGROUND OF THE INVENTION Hydraulic fracturing of earth formations hasbeen carried out for many years. It increases the drainage area into thewell by extending the relatively low pressure zone of the well backthrough the fracture adjacent the rock matrix which is to be drained.Ordinarily this formation fracturing is carried out by confining in azone in the well a liquid having reduced tendency to filter into theformation compared with crude oil or water. The hydraulic pressure onthe liquid is built up until fracturing occurs. Continued pumping athigh pressure causes the fracture to extend as long as the leak awayrate is insufficient to cause the pressure at the tip of the fracture todecrease below the breaking stress of the particular rock involved.

Experimentally it was found that the breaking strength of layered earthstrata was not constant and was in fact a function of the leak away rateof the fracturing liquid. Thus a material having an extremely retardedfiltrate rate against formation rock, such as an extremely viscousliquid, ordinarily causes the rock to fail axially, to produce cracksthe plane of which at least approximately passes through the axis of thebore; a so-called vertical fracture.

Ordinarily the material lying on each side of the fracture so created isessentially unchanged mechanically from its condition before fracture,and accordingly, drainage into the fracture of any valuable liquidcontent of the porous rock (such as oil) depends upon the pressuredifference between that in the rock matrix and that in the fracture, andon the permeability, i.e., the flow characteristics of the formation.Thus while the normal fracturing process described above works well inmost cases in oil, gas, or water wells, the expected improvement isleast in the case of extremely low permeability formations, such as, forexample, the Anona Chalk Formation of Texas and Louisiana. For example,in the well-known Pine Island Field, wells drilled around 100 feet awayfrom a well which had been producing for years and which has beenfractured in this Anona Chalk Formation frequently had essentiallyinitial reservoir pressure in the pay zone. This indicates that thepreviously known fracturing techniques have been inadequate in drainingsuch a rock matrix at any substantial distance from the fracture.

Under these circumstances I believe it is desirable to detonate anexplosive in the drainage fractures of a well, which both increases thetotal effective penetration of the fracture, but of considerably moreimportance, increases the number of fractures by tending to cause thematerial adjacent the original fracture to be rubbled by the explosion,which exposes previously untapped areas to considerably greater pressuredifferentials.

I prefer to do this by injecting a unique liquid explosive into fractuesand fissures in the pay zone, either natural or artificial, so that theliquid before detonation extends a considerable distance from the bore.The explosive in the bore is then detonated. Of course, such liquidexplosive necessarily must have the property of explosive propagation inan elongated and essentially thin, sheet-like form. Also it desirablyshould have low sensitivity prior to detonation so that it has extremelylittle tendency to detonate while being transported to the rig or whilebeing pumped into the well. Otherwise excellent liquid explosives havecaused difficulty on this score.

I am aware that various liquid explosives have been suggested generallyfor the use I propose. For example, the Bureau of Mines has experimentedwith fracturing rock outcrops in which desensitized liquid nitroglycerinwas displaced into the fracture system and detonated. Subsequent welldata obtained with a downhole camera on this very shallow well (of theorder of 625 feet) indicated localized casing damage in the perforatedinterval, and extensive fracturing. However, liquid nitroglycerin, evenwhen desensitized, is not a material handled with ease by the customaryservice companies. Also, it has a viscosity at field temperatures notfar above water and hence can leak away into the formations.

1 am aware that Chesnut in US. Pat. No. 2,892,405 has disclosed andclaimed hydraulic fracturing essentially equivalent to that which I wishto carry out, using sensitized nitromethane which is pumped into cracksand fissures adjacent a bore and detonated from the bore. This is a verymuch safer type explosive. How- I ever, nitromethane is a very lowviscosity material, typically only a few centipoises, as measured forexample with the Fann VG Meter Model 35. If maintained in the formationfor any ordinary period of time it will leak off into the rock matrixand the desired results cannot be obtained. Furthermore, nitromethane isa sufficiently low viscosity liquid so that it cannot be used forcreating additional rock fractures before the detonation, while still inliquid form without pumping at extremely high rates.

The Laurence US. Pat. No. 3,239,305 is also in the field of liquidexplosives. The inventor never teaches or contemplates use of athickening agent. He does mention incorporating into an explosive usingsensitized nitromethane, conventional solid diluents or fillers, such assawdust or diatomaceous earth or liquid diluents such as alcohol orbenzene (Column 1, lines 62-65.) These materials are in no sensethickening agents and do not reduce the fluid loss of the liquidexplosive, as specifically pointed out in applicant's broadest claim.One can convert the sensitized nitromethane (a low viscosity liquid) toa solid of the nature of dynamite by the use of the conventional soliddiluents or fillers, but one does not obtain a thickened liquidexplosive with substantially reduced fluid loss to permeable formations,as taught and claimed by Applicant.

Various experimenters at Dow Chemical Company have described and claimedusing a slurry of a liquid explosive for hydraulic fracturing purposesin earth formations. Such material is found in US. Pat. Nos. 2,867,172Hradel, 2,992,912, Hradel and Staadt, and 3,104,706 Eilers and Park. Inthe first of these, the solid in the slurry is the explosive itself,such as ammonium nitrate, TNT, RDX, and the like. Properly this shouldnot be called a liquid explosive, since the liquid itself serves only asa carrying agent. It has no explosive property. The same comment can bemade on the second patent in which the solid is ammonium nitrate, thoughin this case the liquid (an ammoniacal solution) is essentiallysaturated with ammonium nitrate. In the third, at liquid explosive isused, a mixture of two liquids, one of which is an oxidizing and theother which is a reducing agent. The solid in this case is a finelydivided solid fuel, such as asphalt, carbon or tar.

Other U.S. Pats. from this same source include No. 3,075,463 Eilers etal., No. 3,336,981 Barron et al., and No. 3,336,982 Woodward et al. Thefirst of these teaches use of a liquid explosive including two liquidswhich are respectively a reducing agent and an oxidizing agent, togetherwith an emulsifying agent for causing these materials to stay togetherand not separate in the well. U.S. Pat. No. 3,336,981 comments that anumber of the earlier patents involved hypergolic mixtures whichpresented hazardous problems in the surface mixing involved. It teachesthat a strong oxidizing agent, for example concentrated nitric acid orfuming nitric or sulphuric acid can be gelled with a specific gellingagent, such gel being injected into the well and the fractures followedby a spacer liquid and then by a liquid fuel which forms a hypergolicmixture when in contact with the acid. The materials mix in theformation, resulting in some cases in spontaneous detonation. Thematerials also may be detonated by an igniter means. In the last ofthese patents, U.S. Pat. No. 3,336,982 Woodward et al., a fuel and anoxidizing agent are pumped into well tubing and hence into a formationwith suitable spacer plugs between them, part of the fuel beingrelatively reactive and the other part being relatively active so thatdetonating conditions are found in the formation and a series ofspontaneous explosions ensue.

Practical difficulties were experienced with these systems either inthat initial detonation was difficult or hazardous, or more frequentlythat when the explosive liquid was spread out into a formation fracture,either natural or induced, the detonation did not propagate through thefracture and left considerable undetonated material. Put another way,the critical detonation diameter of the explosives (which measures thepropagating ability of explosive in a sheet form) is at least asubstantial fraction of an inch. Since ordinarily such fractures are ofthe order of V; inch or less, improper detonation or no detonation atall was found in these formation fissures. Accordingly, a materialhaving a critical detonation diameter of the order of 1/32 inch ispreferable.

It is desirable to develop a liquid explosive having a very low criticaldetonation diameter, little tendency to leak away into the formation,and combining both low tendency to detonate accidentally with highreliability and high generation of energy per unit volume of explosive.

In the description of my invention there is reference tomicroencapsulation. A procedure for such encapsulation was given inAdvances in Microencapsulation Techniques" by Flinn and Nack, BattelleTech. Rev. V. 16, n.2, Feb. 1967, p.2. The article Out of packaging DryLiquid no author given, Modern Packaging, V. 39, n. l 0, June 1966, p. l22, teaches encapsulation of drops of liquid such as floor polish andcleaner in plastic, the plastic capsules dissolve to release the liquid.A historical review of encapsulation is given in MicroencapsulatedCatalysts and Resins by Hanny, Haber and Peters, Society of PetroleumEngineering, Reinforced Plastics, Sept. 1966, p. 27, giving reference tothe breaking of liquid or solids-containing capsules by heat, pressure,or both, so that the capsule contents can react chemically with thesurrounding medium.

U.S. Pat. Nos. 3,264,038 and 3,302,977 describe methods for hard-coatingparticles, the coating being removed by dissolving in the surroundingmedium.

SUMMARY The liquid explosive compositions I have invented are based onthe use of a major amount of nitromethane (mononitromethane, CH N0sometimes sensitized with a few percent (based on weight of thenitromethane) of a basic amine which can form salts with weak organic orinorganic acids. To this is added a thickening agent which greatlyincreases the dehydration time of the liquid explosive (this is ameasure of filtrate rate) or produces high apparent viscosity. To thisis preferably added an appreciable amount of powdered aluminum, whichacts as a fluid loss control agent, at times as a sensitizer, and whichcertainly enhances the energy generated per unit volume of explosive. Ialso may add approximately half again as much by weight of ammoniumnitrate as the powdered aluminumj This may be either added directly ormay be added as a concentrated aqueous solution which is emulsified intothe other materials, in which case the fluid loss into the formation isstill further reduced.

i prefer to encapsulate the sensitizer in quite small capsules of asolid only slowly soluble in nitromethane and preferably at least onlyvery slowly soluble in the sensitizer itself. The capsules are mixed inthe other components just before pumping into the well so thatdissolution of the capsules and mixing of the sensitizer with thenitromethane occurs after the liquid is at least far down the well, andperhaps is already out in the fractures extending from the bore. Theprocedure of making such small capsules (major dimension of the order of0.01 inch) is called microencapsulation" and is already well known, aspreviously discussed.

DESCRIPTION OF THE PREFERRED EMBODIMENT The base material employed inthis liquid explosive is mononitromethane, CH N0 To this is added athickening agent, the primary function of which is to increaseconsiderably either the dehydration time of the resultant liquidexplosive or the apparent viscosity of the liquid explosive. In thisconnection, the dehydration time" is defined as the time required forair under psi pressure to blow through a filter paper closing the bottomof a cylinder originally containing a 600 ml sample of the liquid undertest, using the test apparatus and procedure described in APl Code No.29. This dehydration time for my liquid explosive should be at leastthree minutes. More preferably the consistency of the liquid explosiveshould be such that in 30 minutes liquid is still being forced throughthe supported filter paper. In this case one refers to the volume ofcollected liquid in 30 minutes as the AP! fluid loss. I prefer an AP]fluid loss of not more than 100 ml.

The apparent viscosity of the liquid explosive made in accordance withmy invention is at least 30 centipoises and may be as high as severalthousand. Thickened liquid explosives are not ordinarily simpleNewtonian liquids, i.e., the rate of shear is not usually directlyproportional to the shear stress. This is why one measures apparentrather than actual viscosity, i.e., the viscosity measured is a functionof the apparatus used. A commonly used viscosimeter in petroleumproduction laboratories is a Fann viscosimeter, and, accordingly, theapparent viscosity as specified here is determined by this standardpiece of laboratory apparatus. The greater the viscosity of this liquidexplosive (as long as it is readily pumpable under field conditions) thebetter. As mentioned above, apparent viscosities of at least 30centipoises are desired and preferably from at least l00 to severalhundred centipoises is preferred, measured at ordinary room temperature.Viscosities as high as several thousand are still in the usable range,particularly when the liquid explosive is to be pumped at a low linearvelocity, which I ordinarily prefer as involving the least risk ofpremature explosion. When such a high viscosity liquid is placed infractures adjacent the bore of a well the leakaway rate is quite lowcompared to that from ordinary nitromethane or sensitized nitromethane,which has an apparent viscosity in the range of 0.6 centipoise.

Several materials have been found satisfactory for thickening agents fornitromethane. One can use, for example, nitrocellulose (dynamite grade)such as that identified as DuPonts D-2 Pyro which will usually beemployed in a concentration of about two to about five percent byweight, based on the weight of the nitromethane. (All subsequentreference to weight percent is in terms of the weight of thenitromethane.) A second thickener which can be employed is a mixture oftwo materials. One is known commercially as Gantrez An-l 39, which isdefined more specifically as the water soluble copolymer of ethylvinylether and maleic anhydride, having a specific viscosity from 1.0 toabout 1.4 in a 1 percent solution in methyl ethyl ketone at C. This ismanufactured by General Aniline & Film Corporation. The other materialis known commercially as Polyox, defined as long chain polymers ofethylene oxide, having a molecular weight in the range of one million toten million. This mixture should have at least 40 percent of eitherconstituent and preferably should be in an approximately 50:50 mix. Useof approximately one-half to two weight percent of such mixture (basedupon weight in relation to the nitromethane) produces a thickened liquidexplosive having an API fluid loss in the range specified above.

A third kind of thickening agent for nitromethane which can be employedis either Bentone 38, or a very close equivalent Bentone 34, both ofwhich are manufactured by National Lead Company. Bentone 38 is anorganic derivative of a special magnesium montmorillonite used forthickening or gelling organic liquids. It produces thixotropic gels withhigh efficiency. It is supplied as a fine, creamy-white powder with aspecific gravity of 1.8 and a bulking value of 15.0 lbs/gal. Bentone 34is a finely divided light cream-colored powder which is chemicallydimethyldioctadecyl ammonium bentonite, with a specific gravity of 1.8,bulking value of 15.0 lbs/gal, fineness, less than 5 percent on a 200mesh screen, and water content of less than 3.0 percent. It also has theproperty ofswelling in liquid organic systems. Use of about one to fiveweight percent of either of these materials yields a thickened liquidexplosive having the above APl fluid loss characteristic.

If the well in which the liquid explosive is to be employed has atemperature in the pay zone of the order of around 200 F. or higher, thethickened nitromethane liquid explosive may be used withoutsensitization. In such circumstance the thickened nitromethane is pumpedslowly into the well and into the fractures extending into the pay zoneand detonate. Under the elevated temperature the detonation is easilyaccomplished and the detonation will progress throughout the earthfractures loaded with the liquid explosive. Only if the temperature inthe pay zone is distinctly less than around 200 is there a necessity toresort to sensitization of the nitromethane.

The nitromethane may be sensitized prior to thickening. As mentionedabove, this can be accomplished with several organic amines, andspecifically, those sufficiently basic to form salts with a weak acid,either organic or inorganic. I prefer to use ethylene diamine. Otherswhich are found to be very effective are diethylene triamine andtriethylene tetramine. Of less effectiveness areZ-amino-methyl-l-propanol furfurylamine, triethylamine, and pyridine.The quantity of the sensitizer to be employed may be from zero to fivepercent, the smaller quantities being used when the liquid explosive isto be employed in a formation having a temperature of the order of 200F. or greater. Ordinarily one employs a concentration of these amineswhich when mixed with the nitromethane will produce an increase in gapin the ordinary USBM card gap sensitivity test, well known in explosivetesting, of the order of at least 300, but not more than about 450 mils.For comparison, the fuel-oil-ammonium nitrate mixture now being widelyused as a safe explosive has a gap value greater than 1,000 mils, andnitromethane by itself has a gap value of essentially 200 mils.

In order to prevent the liquid explosive from being sensitive at thetime it is injected into a well, I can add the sensitizer, such as anamine or hydrozine, to the other liquid components in an encapsulatedform. In this case the sensitizer is encapsulated in a solid materialthat is slowly soluble in the major liquid component, for instancenitromethane. Until the capsule dissolves, its contents are separatedfrom the other liquid components. In the absence of mixing of thesensitizer with such components, there is an extremely low possibilityof premature detonation, and the liquid explosive containing theencapsulated sensitizer can be placed quite safely in the well. Themajor dimension of such capsules should be quite small, of the order ofthousandths of an inch preferably, and ordinarily not more than aboutthirty thousandths of an inch maximum. The wall thickness is of theorder of one quarter to one sixth the maximum capsule dimension. Sincecapsules of such dimensions may be easily forced by flow of the liquidexplosive into the fractures connected to the bore of the well, thecapsules are distributed by this flow to the zones of interest, where bysubsequent solution of capsules in the liquid, the liquid explosive issensitized and ready for detonation as the shock wave from explosion inthe bore propagates through the explosive placed in the fractures.

Even if the capsules dissolve in the bore before the fractures arereached, if the dissolution does not occur before the capsules are atleast some distance below the well head, say a few hundred feet or so,safe pumping of the liquid explosive has been achieved.

Numerous solids for encapsulation purposes can be employed, the basicrequirements being low solubility in nitromethane and very lowsolubility in the sensitizer. I prefer to employ such solids ascellulose acetate, cellulose triacetate and the vinyl resins, such aspolyvinyl chloride or polyvinyl acetate. The procedure for encapsulationhas been discussed in the references given above. No unusual techniqueis required.

These capsules are made up in advance of use, and are added in themixing of the liquids at the well or lubricated in at the wellhead. Thisgives minimum capsule exposure to the nitromethane component above thewell tubing and tends to insure that there is minimum possibility of thesensitizer contacting the liquid components until they are at leasthundreds of feet below ground.

A thickened explosive compounded as described above works quitesatisfactorily in the explosive fracturing of wells. It combines a verylow tendency for accidental detonation with high reliability, and a verygreat decrease in tendency to leak off into the surrounding permeablerock formations underground. rdinarily in use it is placed in an alreadyexisting fracture or fractures in the well which may and customarily arealready filled to a degree with some type of propping material which maybe ordinary sand or special proppant as already taught in the fracturingart.

I prefer to add to the thickened liquid a substantial quantity of finelydivided aluminum. Powered aluminum is satisfactory. Up to 25 weightpercent can be used, preferably from 10 to weight percent. The aluminumacts in the explosive three ways. it tends to decrease leakage from thefractures to the adjacent rock formations since it builds a filter cakeon the fracture walls. It acts to a degree as a sensitizer. Third, itenhances the energy generation upon detonation since the oxidationreaction is highly exothermic.

The reaction occurring at the time of the explosion can be made stillmore exothermic, and hence greater explosive force can be exerted on theformation walls, if there is added to the above mix of nitromethane, asensitizer, a thickening agent, and powdered aluminum, and appreciablequantity of ammonium nitrate. This ammonium nitrate decomposes at timeof detonation to release oxygen which reacts with the aluminum, and, ofcourse, with the nitromethane itself. Preferably the weight of ammoniumnitrate added should be between about I to 2 times the weight of thepowdered aluminum added to the nitromethane, typically about 1.5 times.

Since the mixture contains a thickening agent, the viscosity of themixture is considerably above that of the base ingredients, and as aresult, both the aluminum powder and dry powdered ammonium nitrate maybe added directly to the basic materials, thus forming effectively aslurry. However, I have also found that a more desirable arrangement foradding the ammonium nitrate is to make a concentrated aqueous solutionof this material, using as little excess water as possible, and emulsifythis aqueous solution into the nitromethane mixture by the use of a verysmall amount of a suitable surface active agent. This arrangement offersthe additional benefit that, as an emulsion, the resultant mixture ismore viscous for the same amount of primary thickening agent than amixture in which no water was employed.

A large number of surface active agents are known which emulsify anaqueous solution in an oily material, such as sensitized nitromethane.Thus, for example, the isopropyl amine salt of dodecyl benzene sulfonate(sold for example by Atlas Chemical Company under the trade name of0-3300) may be used as the emulsifying agent in a concentration of theorder of to 3 percent of the weight of the aqueous solution of ammoniumnitrate. Another suitable arrangement consists in using a nonioniccomplex mixture which is a surface active agent and has ahydrophile-lipophile balance (HLB) roughly of the order of 5. Forexample sorbitan monosterate (HLB of 5.9 to 4.7), or sorbitan monooleate(HLB 4.3) which are sold by Atlas Chemical Company under the designationof Span 62 or 60, and Span 80, respectively, are such materials. Theymay be used in a concentration in the order of '3 to approximately 25percent by weight of the aqueous liquid. Many other such surface activeagents could be employed, the only criteria being low net cost andrelatively powerful emulsifying ability.

The following examples cover compositions made in accordance with myinvention. In these explosives the weight percent of all ingredients,other than the nitromethane, is stated in terms of percentages by weightof the nitromethane.

Composition No. l

0 to 5 percent Ethylene diamine (sensitizer 0 to 25 percent Powderedaluminum, (sensitizer and fluid loss control agent) Composition No. 2

Primary Ingredient About 1 to about 5 percent 0 to 5 percent 0 to 25percent Nitromethane, Bentone 38 (thickener) Ethylene diamine(sensitizer Powdered aluminum, (fluid loss additive and sensitizer)Composition No. 3

Primary Ingredient About 0.5 to 2 percent Nitromethane Gantrez AN'polyox5050 mix (thickening agent) Ethylene diamine (sensitizer Powderedaluminum,

(sensitizer and fluid loss additive) 0 to 5 percent b 0 to 25 percentComposition No.

Nitromethane Primary ingredient Nitrocellulose, dynamite grade About 2to about 5 (thickener) percent Ethylene diamine (sensitizer) Powderedaluminum (sensitizer and fluid loss additive) Ammonium nitrate, powdered(oxidizer) 0 to 5 percent It) to 25 percent 15 t 37 percent CompositionNo. 5

Nitromethane Primary ingredient Nitrocellulose, dynamite grade About 2to about 5 (thickener) percent Ethylene diamine (sensitizer Powderedaluminum,

0 to 5 percent 10 to 25 percent 30 to 75 percent Anyone versed inmanufacture of liquid explosives realizes that the detailed compositionslisted establishes a range for a large variety of allied compositionswhich can be manufactured. The major emphasis in this whole developmentis to replace the known relatively low viscosity liquid explosiveshaving rather high leakaway rates into permeable formations, with liquidexplosives having sufficient bodying capacity or viscosity so that theyhave low leak-away rates, and tend to remain in place in the fracturesformed in the formation prior to detonation. Accordingly, the samevolume of liquid explosive made as specified above will occupy a largerultimate volume as one cohesive body in the formation prior to theexplosion than the unthickened liquid explosives already known.Accordingly, it will produce a greater shattering effect, whichincreases the drainage area. Also it has a greater average thicknesswhen present in the fracture, thus requiring less stringent requirementson the critical detonation diameter of the explosive.

Incidentally, the critical detonation diameter of the compositions givenabove are not greater than approximately 1/32 inch and are in no casegreater than l/l6 inch, using the conventional rail test. In this testthe liquid explosive is placed in a substantially rectangularly shapedtrough in a steel bar, the depth of the trough being great at one end,progressively decreasing to a minimum depth at the other. In the openrail test, no cover is placed over the filled trough; in the closed railtest a flat steel strip wider than the transverse trough dimension issecurely mounted over the trough, furnishing an almost exact simulationof an earth channel. In either case, the explosive is detonated from thethick end under specified conditions of temperature (ordinarily ambient)and the trough is subsequently examined to determine the minimumthickness of the liquid at which detonation still occurred. Such minimumthickness is in the case of the compositions specified above, of theorder of l/l6 inch to the order of [/32 inch.

On the other hand, it is found that a liquid explosive of any of mycompositions has unusually low sensitivity, i.e., that it does not tendto explode under accidental jars, impacts, moderate heating, etc. Theexplosives made in accordance with my invention are classifiable as ICCClass B explosives.

In use, the liquid explosive as above described is pumped into the welland thence into formation fractures until only a small amount (typicallyoccupying the order of 25 to 50 feet) remains in the wellbore. Adetonator is placed in the explosive in the bore, and the explosive isultimately detonated. Preferably the well is stemmed above the explosivewith a plug set above the explosive, surmounted by at least a hundredfeet of set neat cement. After the explosion the plug is drilled out, ifnecessary, the well cleaned out, and placed on production.

I claim:

1. A liquid explosive composition comprising nitromethane to which hasbeen added from about 0.5

to about 5 weight percent based on weight of said nitromethane o athickening agent capable of increasing the dehydration time (as measuredby the test of API Code No. 20) to at least 3 minutes, and to which hasbeen added a small amount (not more than about 5 weight percent) of anorganic amine sensitizer encapsulated in capsules with solid walls, saidwalls being only slightly soluble in nitromethane and essentiallyinsoluble in said sensitizer.

2. A liquid composition in accordance with claim 1 in which said wallsof said capsules are chosen from the group consisting of celluloseacetate, cellulose triacetate, and the vinyl resins, and in which themaximum capsule dimension does not substantially exceed thirtythousandths of an inch.

1. A liquid explosive composition comprising nitromethane to which hasbeen added from about 0.5 to about 5 weight percent based on weight ofsaid nitromethane of a thickening agent capable of increasing thedehydration time (as measured by the test of API Code No. 20) to atleast 3 minutes, and to which has been added a small amount (not morethan about 5 weight percent) of an organic amine sensitizer encapsulatedin capsules with solid walls, said walls being only slightly soluble innitromethane and essentially insoluble in said sensitizer.